We have a huge garden with many deep borders and having filled in a big pond which we struggled to maintain properly, we also have a new bed (a bit bigger than your plot) there are tall trees on the south side, a hedge on the south side and is fairly open to the north, but because it has a large open area in front of it, it's actually quite sunny. Orientation is really important as is type of soil, you can improve the soil but unless you add lots of topsoil you won't change it that much. We had no option but to get a load of topsoil, it's very sandy and free draining whereas our garden is clay so it's giving me the opportunity to grow plants that won't thrive in the rest of the garden.We find shrubs are our best friend in large areas, there are so many different types and they create a structure, if you choose some evergreens you also get winter interest and also good Autumn colour from your deciduous shrubs. Hardy perenials go in around the shrubs and I plant bulbs for early spring colour towards the front, I fill up gaps in the spring with annuals. Beds like this develop over the years and most years I clear something out and replant, I never get it right straight away but that's part of the fun. I had a lovely collection of penstemon but lost every single one this year because basically my soil is too heavy for them to withstand a really wet winter, but my roses are fantastic because they like a heavy soil. Good luck, a large bed can look daunting but it really does give you scope for having something beautiful, most borders and beds are really too small.